The measurement of gas velocity along the direction of flow is of significant importance in several applications. For example, an accurate determination of wind velocity over oceans or rivers along the direction of the flow is important in predicting tidal patterns, potential weather fluctuations, etc.
The measurement of wind velocity is also of importance in aeronautics such as in wind tunnels to determine the aerodynamics of aircraft designs. Another area where determination of wind velocity is of importance is in airports wherein an accurate determination of wind velocity will increase the safety factor in the landing and take off of airplanes. Another area where determination of wind velocity is of importance is in the field of disaster management. The accurate determination of wind velocity is useful for determining the potential for natural disasters like typhoons, tornadoes and avalanches.
The determination of gas flow velocity as a function of electricity generated due to flow of the gas along a solid material has the added benefit of energy conversion. Thus, the kinetic energy of the gas is converted into electrical energy. This phenomenon has tremendous importance in areas such as medical instrumentation, metrology, pollution detection, automobile industry, aircraft and microscopy.
Several methods are known in the art for the measurement of gas velocity along its flow. For example, one method of low speed flow field velocity determination comprises particle imaging velocimetry, which comprises suspending aerosol particles in the gas. A fast charge coupled device is provided across the planar cross section of the flow in order to image the colloidal particles. The small seed colloidal particles are illuminated using a laser light sheet. The charge coupled device camera electronically records the light scattered from the particles. Analysis of the image obtained enables determination of the particle separation, and thereby the velocity of the particles, which are assumed to follow the path of the flow.
However, this method has several disadvantages. The primary disadvantage is the underlying assumption that the movement of all the colloidal particles assume the direction of the flow. This is not necessarily true in the case of large sized particles or in the case of ver low velocities. Thus, the application of this method is limited to velocities of greater than 2 cm/s. It is thus, also important in this method, to ensure that the particle size is small enough to ensure that the particle follows the flow of the gas but at the same time is large enough to effectively scatter light. The equipment required (lasers, CCD cameras) is also large in size. Another disadvantage is that the method is dependant entirely on image analysis and thereby on the analysis algorithms. Since the particle imaging velocimetry method measures the velocity of the colloidal particles and there is no direct digital signal corresponding to the gas velocity; the flow velocity of pure gas cannot be measured. The method also is not appropriate for systems where optical access is absent. Another disadvantage is that the equipment required such as lasers and charge coupled devices are expensive.
Another method known in the prior art for gas velocity measurement is Doppler velocimetry which comprises measurement of the Doppler shift of scattered light from the gas. The method relies on the fluctuation in the intensity of scattered light received from a gas when passing through the intersection of two laser beams. The Doppler shift between the incident and the scattered light is equal to the frequency of the fluctuation of intensity which is therefore proportional to the component of gas velocity lying in the plane of the two laser beams and perpendicular to their bisector. However, this method also suffers from several disadvantages. The method is operable where the particle velocities are greater than 0.1 cm/sec. This method also requires large and expensive equipment such as a plurality of lasers and digital counters. Another significant disadvantage of this method is that it is restricted to a single point measurement. Similar to particle imaging velocimetry, this method also requires that the particle size be small enough to flow along the gas flow path easily but large enough to produce the required signal above the noise threshold. This method also does not work in systems where optical access to the gas flow path at the measurement volume is absent. Signal level depends on the detector solid angle. As a result while the Mie scattering intensity is substantially better in the forward direction, it is difficult to set up forward receiving optics which remain aligned to the moving measurement volume. Greater noise at higher speed with radio frequency interference is possible. Again, similar to the PIV method, the flow velocity of unseeded gas cannot be measured since there is no direct digital signal corresponding to the gas velocity. This method is appropriate only for gas containing colloidal particles and not for clear gas.
U.S. Pat. Nos. 3,915,572 and 6,141,086 disclose a Laser Doppler velocimeter for measurement of velocity of objects or wind so as to determine the speed or relative speed of the object (such as for example an automobile) or in the case of wind measurement, the true air speed or wind gradients such as wind shear.
Another known method to measure fluid velocity comprises the measurement of heat transfer change using a electrically heated sensor such as a wire or a thin film maintained at a constant predetermined temperature using an electronic control circuit. The heat sensor is exposed to the fluid whose velocity measurement is to be taken. The fluid flowing past the sensor cools the heat sensor which is compensated by an increased current flow from the electronic control circuit. Thus, the flow velocity of the fluid can be measured as a function of the compensating current imparted to the heater by the electronic control circuit.
However, in this method a slight variation in the temperature, pressure or composition of the fluid under study can result in erroneous readings. In order to maintain a relatively accurate measurement from the heat sensor, it is also necessary to provide complicated compensating electronics for constantly calibrating the sensor against any change in environmental parameters. In addition, even such compensating electronics can be subject to error. The sensor generally is operable at fluid velocities of greater than 1 cm/second and not for very low velocities. At low velocities, the convention currents in the gas cause a malfunction in the sensor.
U.S. Pat. No. 6,470,471 discloses a gas flow sensor using a heated resistance wire commonly called a hot wire anemometer. U.S. Pat. No. 6,112,591 discloses a high response, heat transfer detection type flow sensor manufactured using micro-machining technology for IC production. This sensor has an improved efficiency of heat transfer from a heating element to heat receiving (sensing) element by controlling the direction of the gas flow between the elements or by using the characteristics of the fluid flow therein.
It is also known to calculate flow fluid velocity in high viscosity fluids using a plurality of pairs of piezo-resistive pressure sensors across an integrated fluid restriction in order to measure the differential pressure. However, this device measures the volumetric flow rates and not flow velocities. Also, this method is suitable only for measurement of small flow rates.
Yet another method for the measurement of flow velocities comprises the use of rotary flour meters which work on an arrangement of turbine wheels. The motion of the gas through the turbine, otherwise called the rotor wheel, causes the turbine to rotate. The rotational frequency of the rotor wheel depends on the velocity of the gas and is measured using either an electro-optical system or by electronically sensing the square wave pulse generated by magnets embedded in the turbine vanes. The size of the sensor arrangement is also to the order of 50 cm3. The rotary flow meter is suitable for use in cooling systems irrespective of the nature of the gas (clear or seeded) and the sensor can determine if the gas is flowing in the forward or reverse direction.
As can be seen from the above discussion, the various methods known in the art for the measurement of flow velocities suffer from various disadvantages. Both particle imaging velocimetry and Doppler velocimetry require optical access and use lasers. As a result they are not suitable for example, in biological systems. The equipment size is also large rendering it expensive. Thermal anemometry requires large volumes of gases in order to minimize convection currents and generally is suitable only for large velocities. Thus it is not suitable for systems which involve small volumes of fluids flowing at low flow velocity. Rotary flow meters, pressure sensors and vortex flow sensors do not measure the flow velocities directly but rather the volumetric flow rates.
Another important area of investigation is the conversion of energy and energy conversion devices which are economical and possess a long life. Another area where energy conversion devices are required is for supply of electricity for domestic and industrial use. Currently, the demand for electrical energy worldwide is met by one of three sources: nuclear power, thermal power and hydroelectric power. Nuclear power plants require expensive safety equipment and measures in view of the potential for radiation leakage. Thermal power plants use fossil fuels, which result in the attendant problems of pollution and also suffer from reduced supplies due to depletion of fossil fuels and oil. Hydroelectric power requires large dams to be constructed and is completely dependant on water flow in a river or any other water source. The equipment required is also expensive and occupies a large area. Of the various devices and methods of flow velocity measurement, only one, namely rotary flow sensor can actually also generate electricity due to the action of the fluid flow across the turbine blades. However, the magnitude of the power generated in relation of the size of the device renders it unsuitable for use for large scale energy conversion.
It is also known in the art to generate electricity by wind power comprising windmills which utilize the flow of wind to generate electricity by the turning of turbines attached thereto. However, this method has the disadvantages of requiring a high degree of capital investment and space.
Ghosh et al, Science, 2999, 1042 (2003) and U.S. patent application Ser. No. 10/306,838 teach that the flow of liquids over single wall carbon nanotubes results in the generation of electricity in the flow direction and can be utilized for the measurement of liquid velocities. This disclosure also teaches that the induced voltage has a logarithmic dependence on flow velocity over the entire range of velocities 10−7 to 10−1 m/s. It is believed that this is due to the direct forcing of the free charge carriers in the nanotubes by the fluctuation of the Coulombic field of the liquid flowing past the nanotubes in terms of pulsating asymmetric ratchets. This results is a sub-linear dependence of induced voltage on the flow velocity. However, this phenomenon was specific to one-dimensional nature of the carriers in the nano-tubes and was absent in other solid material such as graphite or semi-conductors. Kral & Shapiro, Phys. Rev. left., 86, 131 (2001) teach the generation of electric current and voltage by the transfer of momentum from flowing liquid molecules to the acoustic phonons in a nanotube as the phonon quasi-momentum, which in turn drags free charge carriers in the nanotube. This results in a linear relation between the induced current/voltage and the flow velocity.
In view of the large abundance of wind and other gases, it is desirable to develop a method and a device whereby the flow of gases can be utilized to convert energy irrespective of the scale of energy required as well as measurement of flow velocities of a low range irrespective of the nature of the gas.